The invention is concerned with an explosive device utilized to fracture earth strata adjacent a borehole to thereby increase the permeability of the earth in order to improve the flow rate of oil or gas being recovered from the earth.
The invention is particularly directed to an earth fracturing apparatus of the type described in U.S. Pat. No. 4,160,412 which was granted to William H. Snyder, Ralph E. Williams and John Wisotski on July 10, 1979. The device therein disclosed comprises an elongated cylindrical casing having a centrally located charge and additional charges positioned at opposite ends of the casing. The central charge is detonated to cut through the casing whereby the explosive forces act on the surrounding strata of the earth. Either simultaneously with the central charge detonation or after a slight delay, the end charges simultaneously are detonated to reinforce the explosive forces of the central charge.
In order to achieve proper detonation of the several charges, it is necessary that the integrity of the charges and their detonation train be maintained. The present invention provides means for accomplishing this objective by positively supporting the detonation train within the casing and preventing the entry of water or foreign substances into the casing.